Although I have read many soap-making books (checked most of them out at the library), I must say that everything I really and truly know about soap-making I learned from reading and interacting with other experienced soap-makers here at SMF and also on a few other forums in real time, and then putting it all into practice. And let me not forget one of the best teachers of all- learning from the myriad of mistakes I made along the way.
When it comes to books on soap-making, buyer beware. Some soap-making books are good and can be quite useful, but even the best ones can take you only so far since they are snapshots frozen in time, i.e., they are only as good as whatever reliable soaping info was known and included in it at the time of its printing.
For example, one of the books that I read instructed you to pour the water into the lye instead of the other (proper/safe) way around; and several other books erroneously instruct you to refrain from adding your special super-fatting oil to your CP until it has come to trace so that it will remain untouched by the lye in your finished soap (a myth that has since been thoroughly busted in more recent years).
Personal biases can muddle things, too: for example, the author of an otherwise fairly good book that I read strongly advised against using lard or tallow in soap because (in her opinion) they are inferior fats that don't make good soap. I found out later while reading that she was a vegetarian.
All of the above are the reasons why I'm a big advocate of reading the forums over books, and even over watching Youtube videos for that matter. Books, as well as many youtube videos are limited to the experiences/opinions/biases of one person, while on a forum there is up-to-date info and wisdom among many to be gleaned.
Of all the books I've read, the most helpful to me has been 'Scientific Soapmaking' by Dr. Kevin Dunn, which he wrote as curriculum for his chemistry students at the university that he teaches. I must warn you that it is not a how-to' book with lots of pretty pictures and/or recipes with which you can cozily curl up on the couch to read with a cup of tea. Instead, it takes you behind the scenes into the molecular underbelly to be able to gain a better understanding of what takes place inside soap though all its stages.
IrishLass